"All four candidates being considered by BIPM are going to optical carrier frequencies," said principal investigator and NIST Fellow Jun Ye. "We like our strontium-based atomic clock because it is based on many neutral atomsall working together to get your precision level up."

Today's standard atomic clock uses a cigar-shaped cloud of cesium atoms that is confined by infrared laser beams directed at right angles to each other. Cesium atoms are pushed into a ball, slowing down their movement and cooling them to near absolute zero. Two vertical lasers toss the atoms upward into a microwave cavity. Finally, all lasers are turned off and the atoms fall back to the bottom of the cavity like a fountain.

When atoms hit the bottom of the cavity, the process repeats itself. By tuning the microwaves in the cavity to the cesium atoms' natural resonant frequency (9.19 GHz), the fountain can be made to oscillate at exactly one second.

The strontium-based atomic clock also uses lasers to confine and cool atoms, but the natural resonant frequency is locked in with a new measurement methodology called the "optical frequency comb." The technique was co-invented by NIST Fellow and Nobel laureate John Hall. The optical frequency comb is a laser-based spectroscopy technique that uses interference effects to generate an ultraprecise, time-based reference consisting of femtosecond-length pulses.

Ye's group used these pulses to precisely tune laser frequency to match the natural resonant frequency of strontium atoms (430 THz). The result, the researchers claim, is a more accurate atomic clock.

The optical lattice uses a one-dimensional array of 100 pancake-shaped wells created with a near-infrared laser. Each contains 100 strontium atoms. The light lattice slows atoms and constrains their atomic motion more than a standard cesium atomic clock, restricting possible mistakes to the atomic frequency contributed by atomic motions.

Another candidate for the next generation of atomic clocks, also being developed at NIST by physicist Jim Bergquist, is based on a single mercury ion, which provides a higher frequency time base than strontium-based atomic clock. One drawback is that its signal is weaker due to the use of a single quantum absorber.

"Our strontium-based atomic clock uses tens of thousands of strontium atoms to create a very strong signal," said Ye. Other NIST researchers are experimenting with atomic clocks based on vibrating ytterbium atoms.

The strontium-based approach could also serve as a storage mechanism for future quantum computers, Ye said.

Improved atomic clocks could enable more precise navigation and positioning systems, better communications networks and could help researchers better test theories about the fundamental laws of physics.